The structures of the human eye transmit an image to the retina based on photons absorbed from the visual field. The retina contains five different cell types, organized in laminar fashion. At the back of the retina, furthest from the cornea, are a plurality of photoreceptors that convert light into electrochemical signals. Photoreceptors exist in two varieties: rod photoreceptors and cone photoreceptors. Rod photoreceptors have a long, cylindrically shaped outer segment with membranous disks that are stacked with photopigment. Cone cells have a shorter, more tapered outer segment with fewer membranous disks. The rods are much more sensitive to light than cones and mediate most vision at night or in low light. In contrast, the cones are differentially sensitive to varying wavelengths, and mediate color vision.
The electrochemical signals are relayed from the photoreceptors through the bipolar cells to the ganglion cells. The ganglion cells gather information and send it to the brain through the optic nerve. The innermost layer is the ganglion cell layer, which is the location of the ganglion cell bodies. The inner nuclear layer contains the cell bodies of the bipolar, amacrine and horizontal cells and the outer nuclear layer contains the cell bodies of the photoreceptors. The inner plexiform layer contains the connections between the bipolar, amacrine and ganglion cells. The outer plexiform layer contains the connections between the photoreceptors, horizontal cells and bipolar cells. The outer segments of the photoreceptor cells border on the pigmented epithelium, which absorbs excess light at the back of the retina.
Just like the rods and cones, whose structure and function are oriented entirely toward converting light energy into nerve impulses, every other type of cell in the retina is located and connected to perform some initial step in the processing of visual information.
While the other neurons in the retina emit only graduated electrical potentials, the ganglion cells are the only ones that send out neural signals in the form of action potentials. When it is considered that it is the ganglion cells' axons that form the optic nerve and thereby transmit information from the retina over large distances, the significance of the generation of action potentials in these cells becomes apparent. These potentials are generated spontaneously; and it is the frequency at which they are discharged that is increased or decreased by the appearance of light in these cells' receptive fields.
Though most ganglion cells have either ON-centre OFF-surround receptive fields or the reverse, there are other criteria that define other categories. On the basis of overall appearance, neural connections, and electrophysiological traits, at least three such categories of ganglion cells have been distinguished in retinas. However it is believed that at least eighteen different categories of ganglion cells exist in a human retina.
Intermediate cells such as bipolar cells, amacrine cells and horizontal cells convey the information received by the photoreceptors to neurons called ganglion cells. The human eye contains about 1.2 to 1.5 million retinal ganglion cells. As discussed above, there are three major types (subtypes) or categories of ganglion cells classified by their structure and function. These cells, the magnocellular cells (m-cells), parvocellular cells (p-cells) and koniocellular cells (k-cells) each having a unique role in visual processing.
The small parvocellular (or “p-cells”) ganglion cells (from the Latin parvus, meaning “small”) represent about 90% of the total population of ganglion cells. Large magnocellular (or “m-type”) ganglion cells (from the Latin magnus, meaning “large”) account for about 5%. Non-m, non-p ganglion cells, which have not yet been well characterized, account for the remaining 5%. These non-m, non-p cells include k-cells.
M-cells receive signals from a large number of photoreceptor cells. They have fast conduction velocities resulting in quick propagation of nerve impulses over a relatively large receptive field. The m-cells process images with low spatial resolution, but a fast temporal resolution. Furthermore, the m-cells demonstrate association with regions of the brain responsible for motion perception. Although these cells are sensitive to contrast stimulus, they show only weak response to chromatic input.
In contrast, the p-cells are responsible for the processing and visualization of color stimulus. They are generally involved in processing images at a lower conduction velocity and have a smaller receptive field responding to a small number of photoreceptor cells. Particularly, the p-cells show red-green color opponency having responses consistent with the interaction between medium-wavelength-sensitive (M or “green”) and long-wavelength-sensitive (L or “red”) photoreceptor cone cells. The p-cells show sustained response to stimuli and, opposite the m-cells, process images with high spatial resolution and slowed temporal resolution. The p-cells show association with areas of the brain relating to visual acuity and color perception.
The most commonly accepted theory is that m-cells are particularly involved in detecting movement in a stimulus, whereas p-cells, with their small receptive fields, would be more sensitive to its shape and details.
Cells belonging to the koniocellular ganglion pathway have a large visual field and show blue-yellow color opponency. K-cells show responses consistent with excitation from the short-wavelength-sensitive (S or “blue”) and opponent input from a mixture of M and L cones. These “blue-on” cells are thought to derive opponent cone inputs through depolarizing and hyperpolarizing pathways.
Another distinction is essential for color detection: most p-cells and some non-m non-p cells are sensitive to differences in the wavelengths of light. Most p-cells are in fact “single color opponent cells,” which means that the response to a given wavelength at the centre of their receptive fields is inhibited by the response to another wavelength in the surround. In the case of a cell with a red ON-centre and a green OFF-surround, red cones occupy the centre of the field and green cones occupy the surround. The same thing goes for cells with blue-yellow opposition, in which blue cones are opposed to red and green ones. Type M ganglion cells do not have any color opposition, simply because both the centre and the surround simultaneously receive information from more than one type of cone. Also, there are no m-cells in the fovea, which confirms that these cells do not play a role in processing color.
Various methods for determining the function of specific retinal ganglion cell types are known in the art. Current diagnostic tools and methods center on comparison of an individual to population norms to identify disease processes and stabilizing visual acuity based on these norms as well as coordination and timing of vision with bodily movements. However, these diagnostics and methods fail to address the visual processing of an individual as a ratio of retinal ganglion cell function. Furthermore, a correlation between the ratio or level of retinal ganglion function and high visual performance has not been thoroughly explained. Thus, there is a need in the art to determine and alter the ratio of function in the various retinal ganglion cells.